This invention relates to a method and apparatus for rejuvenating the cathode of an electron gun of a cathode ray tube where the usuable cathode emission thereof has dropped to an unacceptable level.
Cathodes of cathoode ray tubes commonly comprise an open end cylindrical body coated with an oxide which emits electrons when heated to an elevated temperature. These cathode structures are generally heated by a filament contained within the cylindrical cathode structure through which filament current is passed to generate the heat transmitted to the surrounding cathode structure. The end wall of the cylindrical cathode structure is mounted in confronting relationship to an apertured electrode referred to as a control grid to which, in the normal operation of the cathode ray tube, a varying amplitude negative signal voltage is coupled which varies the intensity of the electron beam passing through the control grid aperture. The electron emission of the cathode frequently reduces to an unacceptable level after the cathode ray tube has been used for a considerable period of time. A number of techniques for rejuvenating a worn-out cathode have been heretofore developed. However, neither the causes of the wear-out phenomenon of the cathodes nor the processes by which the cathodes are rejuvenated are fully understood.
While several techniques for rejuvenating similar oxide coated cathodes used in small signal amplifying vacuum tubes have been known for many years since the early days of radio, there was little economic value in cathode rejuvenation prior to the need to replace the expensive cathode ray tubes of television receivers. Since the early days of television, however, various methods have come into use for increasing the emission of a poor electron emitting cathode. In one such method, the heater filament volatge is increased to a value above normal voltage by means of a step-up booster transformer which must be premanently attached to the cathode ray tube base. This method provides an extension of useful life varying from 3 months to a year. However, there are some drawbacks associated with the use of this method which have increased with the advent of color television cathode ray tube and recent advancements which supply the heater with direct current derived from a high frequency power supply or the horizontal output transformer, which makes useless the provision of a simple AC operated booster transformer for cathode rejuvenation. Perhaps the greatest disadvantage in the use of this method is the need to maintain for sales to customers a large inventory of differently wired booster units for the various cathode ray tube models. A further disadvantage of using such a rejuvenation method is the increased stress put on the heater filaments by the sustained higher than normal voltages required which increases the risk of heater filament failure and the possible damaging of the cathode or cathodes by the continuous above normal cathode temperatures produced in the cathode or cathodes which do not require rejuvenation.
Another technique for cathode rejuvenation is one which does not involve the permanent attachment of a cathode emmission restoring unit to the cathode ray tube. In this method, the cathode ray tube involved is temporarily disconnected from the chassis circuitry and the control grid of a defective electron gun is temporarily connected to a source of high positive voltage while the heater filament thereof is connected to a normal or above normal voltage to cause a very much greater than normal cathode current to flow, which somehow removes emission inhibiting contaminants. Unfortunately, unless this process is very carefully controlled, the cathode can easily be damaged, and instead of improving performance the emission capability of the cathode is lowered or destroyed. A manually operative switch is generally provided to control the period during which the control grid and heater filament voltages are simultaneously applied. However, the operator has no way of knowing the proper period to close the switch to obtain a safe and successful rejuvenation operation, since the period for a switch closure to obtain a successful and safe rejuvenation varies with each cathode structure involved.
One variation of the rejuvenation technique now being described is to place a current limiting switch in the path of flow of cathode current which switch automatically opens when a given limiting current is reached. A further variation on this technique is to rejuvenate the cathode by progressively increasing the heater current until the cathode current reaches a desired level as viewed on a current indicating meter. When the cathode current reaches such level, a switch is operated to disconnect the heater voltage. It is apparent, however, that in all these variations of the technique of rejuvenation just discussed, there is still a simultaneous application of control grid voltage and heater voltage which can create condition which can damage the cathode and result in less than optimum rejuvenating results.
In the attempt to control the rejuvenation process, it seemed that some kind of timing cycle would have to be employed, or some other means be devised for limiting the total amount of energy available for rejuvenation. A convenient and fairly inexpensive way to accomplish this is to use the capacitor discharge method of rejuvenation. In this way a high voltage can be applied to that a very strong electric field is available for reaching into the cathode below the inactive top surface to get the rejuvenation process started. An extremely large current will flow as emission begins to discharge the capacitor, but the limited amount of energy available in the capacitor reduces the possibility that the cathode will be destroyed. The possibility of damaging the cathode can be further reduced by offering several degrees of rejuvenation, and increasing the amount of energy available in each higher degree. This is accomplished by either increasing the size of the capacitor or the voltage to which it is charged. This technique proved to be an effective way of reducing rejuvenation hazards to picture tubes while still achieving a fairly effective rejuvenation process. However, it was recognized that the older and more hazardous rejuvenation methods had a far longer lasting result.